1. Field of the Invention
The present invention relates to surface acoustic wave devices used for resonators, bandpass filters, and other such devices, and more particularly, relates to a surface acoustic wave device using a rotation-Y plate X propagation LiTaO3 substrate and a method for manufacturing the same.
2. Description of the Related Art
In mobile communication apparatuses such as mobile phones, surface acoustic wave filters have been used, for example, as bandpass filters or duplexers in the RF stage. As this type of surface acoustic wave filter, surface acoustic wave filters using a leakage surface acoustic wave, which filters are each formed of an IDT (interdigital transducer) made of Al and provided on a 36° to 46° rotation-Y plate X propagation LiTaO3 substrate, have been used.
However, this surface acoustic wave filter has a poor frequency-temperature characteristic of −30 ppm/° C. to −40 ppm/° C., and hence it is necessary to improve the frequency-temperature characteristic. Accordingly, in order to improve the frequency-temperature characteristic, a resonator has been proposed having the structure in which after an IDT composed of Al having a normalized thickness of 0.01 to 0.04 is formed on a 36° rotation-Y plate X propagation (Euler angles: 0°, 126°, 0°) LiTaO3 substrate, a SiO2 film is further provided thereon (for example, in Japanese Unexamined Patent Application Publication No. 02-295212). In this structure, by forming the SiO2 film, the frequency-temperature characteristic is improved.
However, when IDTs are formed by using Al so as to produce filters, in order to obtain a sufficiently high reflection coefficient or electromechanical coefficient Ksaw, the IDT must have a relatively large electrode thickness H/λ (H indicates the thickness, and λ indicates the wavelength of a surface acoustic wave) of 0.08 to 0.10 (for example, in O. Kawachi et al., “Optimum Cut of LiTaO3 for High Performance Leaky Surface Acoustic Wave Filters”, Proc. 1996 IEEE Ultrasonic Symp., pp. 71-76).
As described above, since the IDT composed of Al has a relatively large thickness, when a SiO2 film is formed at parts shown in FIG. 25A for improving the frequency-temperature characteristic, as shown in FIGS. 25B and 25C, large steps are formed by the SiO2 film. As a result, cracks may be generated therein in some cases. Accordingly, due to the generation of the cracks, filter characteristics of the surface acoustic wave filter are degraded.
In addition, since the electrode of the IDT composed of Al is relatively large, an effect of covering the irregularities of the electrode surface of the IDT cannot be satisfactorily obtained by forming the SiO2 film, and hence the temperature characteristics may not be sufficiently improved.